I've actually thought of this in the past. It would certainly be interesting. On the surface level, it would work since both Angels and Gems' bodies are essentially made of light, both focus on a protagonist who's lost in the world, and both feature heavy themes of motherhood and family. Going deeper, I think Steven would really be able to help a circa ep 9 or 10 Shinji, who still has hope, but is definitely in need of support that really no one he knows can give. Steven is a natural optimist and mediator, who easily empathizes with others, and is in a similar situation, having lost his mother and being forced to take on her unfinished business.

Many of the other characters are similar as well, Pearl and Misato are both mother figures with poorly-concealed feelings of inferiority, Asuka is similar to both Amethyst and Peridot in personality and mannerisms, and Kaji is like Garnet, in that both are cool-headed natural leaders (although Garnet has started to become more soft and human lately). SEELE and the Diamonds are obvious, and the Corruption Event that ended the Gem War is pretty much the same as Second Impact, just with less of an influence on humans.

I definitely think this could work, given that the writer fully explores the shared themes between both narratives.

I've reached a point where I just generally don't give a fuck anymore. In a good way.